1. Field of the Invention
The invention relates to a permanent magnet for nuclear magnetic resonance image detection, either of the total-body type, for receiving a considerable part of the body, or the whole body, or of the dedicated type, i.e., for detecting nuclear magnetic resonance images only relating to certain parts of the human body.
2. Description of Related Art
In order to obtain spin-echo signals for reconstructing sufficiently faithful images of the part under examination, i.e., valid images, therefrom, the body, or the part thereof under examination must be exposed, in nuclear magnetic resonance machines, to a static magnetic field, having, in the image detection region, certain intensity and homogeneity characteristics. These characteristics are determined by predetermined tolerances.
As extensively discussed in U.S. Pat. No. 5,495,222, an ideal static magnetic field is obtained through a magnetic structure which closes the cavity or the volume for receiving the body or the part thereof on all sides.
Theoretically, the provision of magnets of this type is possible, but they must be considerably large, so as to allow the whole body, for example of a patient, to be received therein. Such a machine is definitely expensive and cumbersome, and involves safety and comfort drawbacks for the patient.
Therefore, prior art machines have cavity-delimiting magnetic structures with at least one open side, with two open sides, generally opposite to each other, or with three or more open sides, like in C-shaped magnetic structures, and in structures having only two opposite and spaced poles and being open all along the peripheral shell of the two poles, except for supporting members, like columns.
Owing to the provision of one or more open sides of the magnetic structure, in that region the magnetic field is diffused out of the structure. Here, the field lines are outwardly swelled, and the field intensity and homogeneity progressively decrease, as the field approaches the open side or sides of the structure. The magnetic field volume having sufficient intensity and homogeneity characteristics for image detection is thus reduced, with respect to the total volume of the cavity defined by the magnet, and with respect to the total volume of the structure thereof.
The immediate solution thereto would be an enlargement of the magnet structure, so as to keep the useful image detection volume as distant as possible from the open side or sides. However, also owing to the necessary relationship between the useful volume and the volume or extension of the body, or the part thereof, to be examined, this solution involves very unfavorable ratios between the total magnet size and the useful image detection volume, i.e., the volume within which the static field has sufficient characteristics for detecting valid nuclear magnetic resonance images. In practice, the magnet structure must have a huge size, and be expensive, both as regards construction and as regards purchasing costs and location of the equipment at the final user's site.
Although this drawback is less serious in the total-body machines, i.e., those having a magnet structure which receives the whole patient body or at least a considerable part thereof, the situation described above is much more burdensome in dedicated machines, i.e., small machines for detecting images of certain localized, specific body parts.
Dedicated machines use small magnetic structures, which must be relatively handy and light. The size compaction of the magnetic structures requires a high ratio between the space requirement of the structure and the useful image detection volume, and between the total cavity volume defined by the magnet and the useful image detection volume. Said useful volume should fill up a considerable part of the total cavity volume.
The problems related to dedicated machines are still more serious because, in order to provide highly flexible machines for several body parts having different anatomic and morphologic characteristics, the magnet structure must have an increasingly great number of open sides.
In certain types of permanent magnets, such as in the magnet described in U.S. Pat. No. 5,495,222, that is a C-shaped magnet with two open sides transverse to the axis of its C shape and one open side parallel thereto, in order to compensate field aberrations there are provided correction means at one open side, consisting in this case of extensions being directed to partially close the opening on the side parallel to the axis. The aberrations at the sides transverse to the axis of the C shape are not compensated, since the magnet is long enough to keep the open sides sufficiently distant from the relevant volume.
The magnet according to this document is of the total-body type, and the patient body is introduced therein in the direction of the axis of the C shape, whereas the partially open side, parallel to the axis, has the function to allow physicians or paramedics to perform operations on the patient.